Help with Bias Circuit - Electronic Circuits Projects Diagrams Free

dfris · 7th January 2009, 02:50 AM

HI,

Im new to this forum and I am in need of some help, Im tring to mod my amp to make it able to accept both el34 and 6l6 tubes. I understand that I need to rebais the amp, but the same company that made my amp Carvin x100b has come out with a reissue of this amp and it has this circuit in the amp. It can handle both types of Tubes. Just flip a switch, and rebias the amp and you can run el34, or 6l6. I would like to do this as well. I have 2 pics of the schematics for both the new version and my version. well my version is for the EL34 tubes, and my Amp is a 6l6 version, but the parts and drawings are the same, except that I have 450 ohm / 2 watt resistors in R113,114,115,116.
And the El34 schematic is done on a PC and you can read it better.

From what I can understand there is only one difference in the 2 schematics after the adjust pot, its that R126 has a switch on that either grounds the resistor or its on open circuit. I would assume that this allows for a voltage drop if its closed (grounded) and a voltage gain if its Open. Seeing it takes almost 10v drop from a 6l6 to el34 tube. I would still have to rebias the amp but this would get you very close.

I am having the most problems with the 2 caps in parallel and the R159 right before the diode on the adjustable version of the schematic. why would they use that? If the diode is making the DC why have a DC blocker before the diode? And whats up with R 159?

Also I noticed they changed the grid resistors from 2.2k (R109,110,111,112) to 4.7k in the new version, Not sure what effect this would have. As I mentioned before I have 470 Ohm resistors on the screen, I am planning on changing mine to 360 ohm 5 watts. the schematics call for 350.5 ohm 5 watts but I dont see the extra ohms being a real problem.

I am thinking a simple swith on R126 would do the trick for me, but I would like some more input on this. Also would i need to add the caps. and R159 on my board to make this work. Also couldnt i just put a .1 UF 630V cap instead of the 2 in parallel?

The New Version: In Black- I have outlined the main points in Red
My Version in Green: Same as mine except for the Screen Resistors.

Thanks for your time, and advice
Daryl

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7th January 2009, 02:50 AM	#1
dfris	Help with Bias Circuit HI, Im new to this forum and I am in need of some help, Im tring to mod my amp to make it able to accept both el34 and 6l6 tubes. I understand that I need to rebais the amp, but the same company that made my amp Carvin x100b has come out with a reissue of this amp and it has this circuit in the amp. It can handle both types of Tubes. Just flip a switch, and rebias the amp and you can run el34, or 6l6. I would like to do this as well. I have 2 pics of the schematics for both the new version and my version. well my version is for the EL34 tubes, and my Amp is a 6l6 version, but the parts and drawings are the same, except that I have 450 ohm / 2 watt resistors in R113,114,115,116. And the El34 schematic is done on a PC and you can read it better. From what I can understand there is only one difference in the 2 schematics after the adjust pot, its that R126 has a switch on that either grounds the resistor or its on open circuit. I would assume that this allows for a voltage drop if its closed (grounded) and a voltage gain if its Open. Seeing it takes almost 10v drop from a 6l6 to el34 tube. I would still have to rebias the amp but this would get you very close. I am having the most problems with the 2 caps in parallel and the R159 right before the diode on the adjustable version of the schematic. why would they use that? If the diode is making the DC why have a DC blocker before the diode? And whats up with R 159? Also I noticed they changed the grid resistors from 2.2k (R109,110,111,112) to 4.7k in the new version, Not sure what effect this would have. As I mentioned before I have 470 Ohm resistors on the screen, I am planning on changing mine to 360 ohm 5 watts. the schematics call for 350.5 ohm 5 watts but I dont see the extra ohms being a real problem. I am thinking a simple swith on R126 would do the trick for me, but I would like some more input on this. Also would i need to add the caps. and R159 on my board to make this work. Also couldnt i just put a .1 UF 630V cap instead of the 2 in parallel? The New Version: In Black- I have outlined the main points in Red My Version in Green: Same as mine except for the Screen Resistors. Thanks for your time, and advice Daryl Attached Thumbnails Last edited by dfris; 7th January 2009 at 02:54 AM.